Vaccines that can block viral transmission will help control the pandemic.Credit: Andrea Fasani / EPA-EFE / Shutterstock
While countries distribute vaccines that prevent COVID-19, studies are underway to determine whether vaccines can also prevent people from becoming infected and transmitting the SARS-CoV-2 virus. Vaccines that prevent transmission can help control the pandemic if administered to a sufficient number of people.
Preliminary analyzes suggest that at least some vaccines may have a blocking effect on transmission. But confirming this effect – and how strong it will be – is complicated because a drop in infections in a given region can be explained by other factors, such as blockages and changes in behavior. In addition, the virus can spread from asymptomatic carriers, making it difficult to detect these infections.
“These are among the most difficult types of studies to do,” says Marc Lipsitch, an infectious disease epidemiologist at the Harvard TH Chan School of Public Health in Boston, Massachusetts. “We are all out there, eagerly trying to see what we can get from the little bits of data coming out,” he says. The results of some studies are expected in the coming weeks.
Stop infections?
While most clinical trials with COVID-19 vaccines show that vaccines prevent disease, some trial results also offer clues that injections can prevent infection. A vaccine that is highly effective in preventing people from getting the infection in the first place would help reduce transmission, says Larry Corey, a vaccinologist at Fred Hutchinson Cancer Research Center in Seattle, Washington.
During the test of the Moderna vaccine, produced in Boston, the researchers collected samples from all participants to see if they had any viral RNA. They observed a two-third drop in the number of asymptomatic infections among people who received their first injection of the two-dose vaccine, compared to those who received a placebo. But they tested people only twice, about a month apart, so they may not have detected infections.
The UK trial of the vaccine produced by the University of Oxford and AstraZeneca collected participants every week and estimated a 49.3% reduction in asymptomatic infections among a subset of vaccinated participants compared to the unvaccinated group.
New York City-based Pfizer and maker of another leading vaccine, COVID-19, says it will start cleaning participants every two weeks in vaccine tests that take place in the United States and Argentina, to see if the injection can prevent infection.
Less infectious?
Vaccines may not stop or significantly reduce the chances of infection. But injections can make infected people less able to transmit the virus or make them less infectious and thus reduce transmission.
Several research groups in Israel are measuring ‘viral load’ – the concentration of viral particles in vaccinated people who later tested positive for SARS-CoV-2. Researchers found that viral load is a good indicator of infection1.
In preliminary work, a team observed a significant drop in viral load in a small number of people infected with SARS-CoV-2 within two to four weeks after receiving their first dose of the Pfizer vaccine, compared with those who contracted the virus in the first two weeks after the injectiontwo. “The data is certainly intriguing and suggestive that vaccination can reduce the infectivity of COVID-19 cases, even if it does not completely prevent infection,” says Virginia Pitzer, model of infectious diseases at the Yale School of Public Health in New York. Haven, Connecticut. The Oxford-AstraZeneca study also found a greater reduction in viral load in a small group of vaccinated participants than in the unvaccinated group.
But whether these observed reductions in viral load are enough to make someone less infectious in real life remains unclear, say the researchers.
Golden pattern
To really determine whether vaccines prevent transmission, researchers are tracking contacts close to vaccinated people to see if they are being indirectly protected from infection.
As part of an ongoing study with hundreds of healthcare professionals in England, known as PANTHER, researchers at the University of Nottingham tested healthcare professionals and the people they lived with for SARS-CoV-2 antibodies and viral RNA between April and In August last year, at the time of the first pandemic wave. They will now retest some of these workers after receiving the Pfizer vaccine, as well as their close contacts who have not been vaccinated, to see if the risk of infection has decreased for close contacts, says Ana Valdes, a genetic epidemiologist at the University of Nottingham. If the risk decreases, it would mean that vaccines are probably preventing transmission, says Valdes.
Other groups in Israel also plan to study families in which a member has been vaccinated. If these people are infected, the researchers can see if they transmit the virus to other family members.
In Brazil, a trial will randomly distribute doses of the COVID-19 vaccine produced by the Beijing-based pharmaceutical company Sinovac to the city of Serrana in stages over several months. This approach can show whether drops in COVID-19 in vaccinated regions also contribute to reducing transmission in unvaccinated areas. That would demonstrate the indirect effects of vaccines, says Nicole Basta, an infectious disease epidemiologist at McGill University in Montreal, Canada.
Studies of larger individuals and populations are needed to see how well vaccines protect against transmission, says Basta. “We really need evidence that spans the entire spectrum.”